Book Image

QGIS 2 Cookbook

By : Alex Mandel, Víctor Olaya Ferrero, Anita Graser, Alexander Bruy
Book Image

QGIS 2 Cookbook

By: Alex Mandel, Víctor Olaya Ferrero, Anita Graser, Alexander Bruy

Overview of this book

QGIS is a user-friendly, cross-platform desktop geographic information system used to make maps and analyze spatial data. QGIS allows users to understand, question, interpret, and visualize spatial data in many ways that reveal relationships, patterns, and trends in the form of maps. This book is a collection of simple to advanced techniques that are needed in everyday geospatial work, and shows how to accomplish them with QGIS. You will begin by understanding the different types of data management techniques, as well as how data exploration works. You will then learn how to perform classic vector and raster analysis with QGIS, apart from creating time-based visualizations. Finally, you will learn how to create interactive and visually appealing maps with custom cartography. By the end of this book, you will have all the necessary knowledge to handle spatial data management, exploration, and visualization tasks in QGIS.
Table of Contents (19 chapters)
QGIS 2 Cookbook
About the Authors
About the Reviewers

Modifying and reclassifying values in raster layers

A very useful technique to work with raster data is changing their values or grouping them into categories. In this recipe, we will see how to do this.

Getting ready

Open the DEM file that we used in previous recipes.

How to do it…

We will classify the elevation in three groups:

  • Lower than 1,000m

  • Between 1,000 and 2,000m

  • Higher than 2,000m

To do this, follow these steps:

  1. Open the Change grid values algorithm from the Processing Toolbox menu. Set the Replace condition parameter to Low Value <= Grid Value < High Value.

  2. Click on the button in the Lookup table parameter and fill the table that will appear, as shown in the following screenshot:

  3. Run the algorithm. This will create the reclassified layer:

How it works…

Values for each cell are compared with the range limits in the lookup table, considering the specified comparison criteria. Whenever a value falls into a given range, the class value specified for this range will be used in the output...